The role of proteases in Cryptosporidium parvum survival, attachment, and invasion are unknown. During the funded period of this MCAP study, further characterization of C. parvum aminopeptidase was accomplished. A specific role in parasitic excystation was identified and several inhibitory compounds were found to be effective in preventing amino- peptidase mediated excystation. In addition, basic biochemical characterization, a partial sequence was obtained from a genomic DNA library. Efforts to obtain the complete sequence and protein expression will continue to investigate the immunoreactivity of C. parvum aminopeptidase.